We are currently developing radioimmunoassays for tricyclic antidepressants. Antisera have been prepared by immunizing rabbits with N-succinylnortriptyline-bovine serum albumin conjugate. The assays require a preliminary extraction procedure to remove interfering metabolites and separate the tertiary amine tricyclics from their corresponding secondary amine metabolites. Using 3H-imipramine as tracer plasma levels of imipramine and desmethylimipramine can be measured down to 10 ng/milliliter, using a 0.1 ml plasma sample. Work is in progress in developing the assay for amitriptyline, nortriptyline, doxepin and desmethyldoxepin. In addition, methods are being explored for the development of RIA's for tricyclic antidepressants that will not require an extraction procedure. This will involve development of antisera that are insensitive to possible interference from metabolites in plasma and that are able, in addition, to distinguish the tertiary amine tricyclics from their corresponding secondary amine metabolites.